Video Stuttering

ok, i3770 3.5 24 gig ram 1333 ( 23.2 usable) win 7 64x ultimate Wacom touch pen interface 2x Ati Radeon 6870 1gb gddr5 ( usually un-crossfired) now, I more than meet the minimum needs for adobe premier, but sometimes, when I am using it, it grinds to a near halt and "becomes unresponsive"...